Why is the poem "Stopping by Woods on a Snowy Evening" so serious?

I can see why someone would characterize this poem's tone as serious although "contemplative" might be the word I would choose.  The narrator, whom I have always assumed was a man, has paused for a moment to appreciate the dark, cold, and quiet beauty of his surroundings, the woods in the snow at night, but we understand that he has a long journey ahead of him, "Miles to go before I sleep" (19), and that he is on a bit of a mission, with "promises to keep" (18) to someone or perhaps more than one someone.   His enjoyment of this brief rest and contemplation of the beauty of his natural surroundings is necessarily brief because of the seriousness of whatever promises he must keep and what seems to be a long, difficult journey.  He cannot linger because of his responsibilities, whatever they may be.  They get in the way, I think, of what might otherwise have been a more lighthearted moment.  This reflects a situation that most of us have found ourselves in, I think. We are able to stop and smell the roses from time to time, but so often, these are brief moments because we have so many obligations in our lives.